Common Names

Pittosporum shield bug, Pittosporum bug

Synonyms

Strachia humeralis Walker, 1868 Antestia orbona Kirkaldy, 1909

Biostatus and Distribution

This adventive shield bug comes from Australia and is found in the North and South Islands of New Zealand. It was first detected in New Zealand in 1950. It occurs on host plants, Pittopsorum species, in gardens and parks as well as in native ecosystems.

Conservation status: Widespread Australian insect that is a minor pest of Pittopsorum species.

Life Stages and Annual Cycle

The Pittosporum shield bug overwinters as adults. Adults appear to gather in suitable overwintering sites and on plants for breeding. They are probably attracted to each other by an aggregation pheromone (volatile chemical). Breeding occurs on Pittosporum trees with unripe fruit and starts in spring, possibly as early as October, and continues until late summer. There are probably several generations per year. The overwintering adult females start laying eggs late spring. Each female lays several batches of eggs over several weeks, perhaps up to 8 weeks. This results in nymphs of all sizes being present at the same time. The resulting adults may mate and lay eggs until early autumn. Sometime during late summer the adult females cease laying eggs. This change is probably induced by a change in day length and perhaps by a combination of day length and temperature.

Eggs are laid in a cluster of up to 14, one for each ovariole. The eggs are a pale blue-green colour when first laid and turn white after a few days. Eggs are about 1 mm in diameter. Eye spots of the nymph are visible just before the egg hatches

Nymphs hatch from the eggs and are seen from November until April. First instar nymphs are like small, black, wingless adults. Nymphs go from one stage to the next by moulting, where the “skin” on the dorsal side splits and the next stage pulls itself out. As the insects progress through the nymphal stages their body and antennae change colour. The first instar is almost circular in outline, and coloured dark brown to black. The head and thorax (middle section of body) are usually black after the first day. The abdomen may be dark brown or black. The areas around the stink glands are black as are the edges of the abdomen. There is a pair of white spots near the front of the abdomen on the upper side. The legs are dark brown or black. The antennae are dark. The second instar is similar in colour to the first instar though there are often white areas on the lateral margins of the thorax (middle section of the body) and the first abdominal segment. It differs in that the head extends forward between the bases of the antennae and that the rostrum is very long and extends beyond the end of the abdomen. The rostrum contains the stylets used for feeding. As the second instar nymph grows, the rostrum no longer extends beyond the abdomen. Also the pair of white spots on the dorsal side of the abdomen turn yellow. The colour of third-fifth instars is similar the second instar.

Small wing buds can be seen on fourth instar nymphs. They are larger and more obvious on fifth instar nymphs. Adults emerge from fifth instar nymphs. In early summer, new eggs may be laid, but egg laying ceases in late summer. Depending on climate there may be 1-3 generations per year.

The length of the lifecycle (time from egg to adult) varies with temperature. In New South Wales, Australia, two entomologists, McDonald and Grigg, reported in 1980 that at fluctuating temperatures ranging from 10-33°C, the time from egg to adult was 41.3 days (range, 40-44 days). The time in days for each life stage were: eggs 5 (4-6), 1st instar 3.9 (3-5), 2nd instar 6.6 (6-8), 3rd instar 7.1 (6-8), 4th instar 9.4 (9-12), 5th instar 9.5 (-11). In New Zealand, Pendergast found that the time from egg to adult was 52 days during February and March in a laboratory.

Walking and flyingThe nymphs and adults have six legs (three pairs) that are used for walking. The adults have two pairs of wings; the front pair is modified as covers for the hind wings. Part of the forewing is coloured brown, while the rest is membranous.

FeedingLike other Hemiptera, the Pittosporum shield bug has sucking mouth parts. The long stylets, special shaped rods, are held in the rostrum. When it feeds, the bug moves the tip of the rostrum to the surface of an unripe fruit or other suitable part of the plant. During feeding the stylets are inserted into the plant. The mandibles hold the rostum in place. The maxillae stylets are inserted further into the plant. They form two tubes, one through which saliva is injects into the plant and a second through which plants juices are sucked up into the insect. Feeding of the Pittosporum shield bug has not been studied, but it probably can use its saliva to digest the tissues of the plant.

Recognition

Pittosporum species, the host plants of the Pittosporum shield bug (Monteithiella humeralis) are also hosts of the Australasian green shield bug (Glaucias amyoti). The adults of the two species are easily distinguished, but the other life stages can be more difficult to tell apart.

Adult Pittosporum shield bugs are brown and smaller than the green adults of the Australasian green shield bug.

Eggs of the two species are of similar size, but differ in colour. Freshly laid eggs of the Pittosporum shield bug are pale green and turn white, while the eggs of the Australasian green shield bug are tan coloured, they can be much paler, more like off-white.

NymphsIn New Zealand the first instar of both species are black and they both sit on their egg shells or stay close so they can be identified by the colour of the eggs.

Second instar nymphs of both species are also similar and are most easily identified by the colour of the egg shell if these are nearby. Pittosporum shield bugs may have a brown abdomen. Both species start with white abdominal spots and as the nymph ages the spots turn yellow. Both species may have white patches on the lateral margin of the thorax and the first abdominal segment.

Later instars of the Pittosporum shield bug are black or dark brown with paler brown areas. Australasian green shield bug third-fifth instar nymphs may be green, though some have a dark head and thorax, and the abdomen is much paler.

Natural Enemies

Eggs of the Pittosporum shield bug may be parasitized by two species of tiny wasps belonging to the family Platygasteridae. Trissolcus oenone (Dodd, 1913), a native species, parasitizes several native shield bugs. Another egg parasitoid, Trissolcus basalisi (Wollaston 1858), was released into New Zealand in 1949 to control the green vegetable bug, Nezara viridula. It also parasitizes eggs of other shield bugs including the Pittosporum shield bug and the Australasian green shield bug. When this wide host range was discovered in the 1960s, it was regarded as beneficial, because at that time protection of crops was regarded as more important than protecting native insects. Eggs parasitized by T. basalis turn black.

Additional Information

Why Stink bugsPentatomidae are often called stink bugs because when handled they emit a strong smell. The nymphs have prominent glands on the upper (dorsal) side of their abdomen, while adults have glands between the bases of their legs. The chemicals may deter predators and cause other bugs to drop to the ground, but some of the chemicals produced may also act as aggregation pheromones.

Research ProjectWhat triggers egg development in the spring and cessation of egg production in the autumn. Pittosporum shield bugs overwinter adults that hide among tree and shrub leaves, but may be seen basking in the sun. It is not known if they feed during this time. At some point in the spring they start feeding and eggs start developing in the ovaries of the females. In Auckland eggs may be found in late November. Overwintering adults could be collected and kept under different day length regimes to determine the environmental trigger for egg production.

In late summer, egg laying ceases. Is this caused by decreasing day length affecting the adults already laying eggs or those that have not yet started? Is the trigger a specific day length or decreasing day length?